This invention relates to an improved method of forming integral asymmetric membranes having ultrathin separation layers.
Loeb and Sourrrajan in U.S. Pat. No. 3,133,132 were first to disclose a method for preparing integrally asymmetric cellulose acetate membranes for desalination of water. The principle of membrane formation disclosed by Loeb and Sourirajan, frequently called the "phase inversion method" has been since extended to include a variety of polymers and separation processes.
The procedure for preparing asymmetric ("skinned") membranes from polymer solutions by phase inversion methods usually comprises the following stages: (1) casting the polymer solution, (2) exposure of the cast solution to air, (3) precipitation of the solution in coagulation media and leaching out solvents, and (4) the optional step of annealing the membrane. Although preparation of asymmetric membranes by direct coagulation without the evaporation step is known, most industrial procedures do include step 2 as a part of asymmetric membrane preparation. The importance of the evaporation step, although well recognized in the art, is highly empirical with optimal parameters such as evaporation temperature, duration of the evaporation step, etc. being determined experimentally for a particular polymer/solvent casting composition. For example, in U.S. Pat. No. 3,724,672, R. L. Leonard and J. D. Bashaw describe preparation of asymmetric hollow fiber membranes for reverse osmosis operations from esters of cellulose by extruding filaments into a controlled evaporation zone then directing the filaments into a cold water coagulation bath following by washing and annealing. Subsequently the use of asymmetric membranes has been extended from liquid-based separations such as reverse osmosis and ultrafiltration to gas separations. Examples of asymmetric membranes for gas separations prepared by phase inversion methods can be found in U.S. Pat. No. 4,944,775; 4,080,744; 4,681,605; and 4,880,441.
Manufacturing of integral asymmetric membranes for gas separations is significantly more difficult than for liquid separations. While the presence of small pores in the membrane can be tolerated or even desired in liquid separations such as desalination, the exceedingly small dimensions of gas molecules combined with low cohesive forces of gases make the presence of even Angstrom size pores in the separation layer unacceptable in gas separations. On the other hand, to achieve high permeability it is essential that the separation layer be kept as thin as possible, since the gas flux is inversely proportional to the membrane thickness. These two diametrically opposed requirements make the manufacturing of asymmetric gas separation membranes exceedingly difficult.
Though the manufacture of essentially defect-free ultrahigh flux asymmetric membranes is known in the art, for example, U.S. Pat. No. 4,902,422 and 4,772,392, it is known to be excessively difficult. Thus it is common in the art to subject gas separation membranes to treatments that effectively eliminate defects that may be present in ultrathin membrane separation layers.
Henis and Tripodi in U.S. Pat. No. 4,230,463 have addressed the presence of defects in asymmetric gas separation membranes by applying a coating. The multicomponent membranes produced by this coating process typically comprise a silicone rubber coating on the surface of an asymmetric membrane made of a glassy polymer. Additional defect-repair methods can be found in U.S. Pat. Nos. 4,877,528; 4,746,333 and 4,776,936.
To attain high levels of gas productivity, membranes have to be prepared with separation layers as thin as possible, preferably below 500 .ANG.. Recently, Kesting et al. in U.S. Pat. No. 4,871,494 have disclosed preparation of high productivity asymmetric membranes with graded density skins. The membranes are formed from casting solutions comprised of a Lewis acid-base complex-solvent system close to the point of incipient gelation.
A different class of gas separation membranes is produced by depositing a thin separation layer on a porous support wherein the material of the deposited layer determines the gas separation characteristics of the overall structure. These composite membranes are sometimes more advantageous since they allow decoupling of the material requirements for a particular gas separation application from engineering design requirements of the porous support. A variety of separation layer materials, support structures and composite membrane manufacturing methods are known in the art. Examples of composite gas separation membranes can be found in U.S. Pat. Nos. 4,243,701; 3,980,456; 4,602,922 and 4,881,954.
It is further known in the art that advanced performance composite membranes are frequently prepared by depositing ultrathin separation layers on support surfaces of uniform porosity (e.g., sharp pore size distribution) and of pore diameter below 1000 .ANG.. Highly asymmetric substrates can frequently provide such advantageous supports.
Procedures for dry-wet spinning of hollow fibers are well known in the art, see for example, I. Cabasso, "Hollow Fiber Membranes", Kirk-Othmer, Enc. of Chem. Tech., 12, Third Ed., 492-518 (1980) and I. Cabasso, "Membranes", Enc. of Pol. Sci. and Eng., 9, Second Ed., 509-579 (1987).
The vast body of knowledge that exists in the field of fiber spinning is also frequently directly applicable and indeed is used extensively in the field of spinning hollow fiber membranes. For example, information on the dry-jet wet-spinning process and the equipment for manufacturing polyamide fibers disclosed in U.S. Pat. No. 3,767,756 can be useful for spinning hollow fiber membranes.
A procedure for preparing fibers from polymer solutions at subatmospheric pressure is disclosed in U.S. Pat. No. 3,842,151 issued Oct. 15, 1974 to Stoy et al. Though the disclosure relates to a method and apparatus for forming fibers, strings, cords, tubings, films, etc., its basic disclosure is directed to the preparation of solid fibers. According to the invention disclosed, a polymer solution is extruded through a spinneret into a tube or shaft whose upper end is sealed against gas flow by a lid connected with the spinneret, and its lower end is placed below the level of a coagulation bath open to the atmosphere. The pressure within the tube between the spinneret and the level of the coagulation bath is maintained lower than the pressure outside the tube or shaft so that the level of the coagulation liquid is higher in the shaft than in the outer coagulation bath. In the spinning process, the polymer solution exits the spinneret into the gaseous atmosphere above the coagulation bath in the shaft, which is maintained at subatmospheric pressure, the fibers then enter the coagulation bath and after passing through the bath are collected. Vacuum means are provided to maintain the level of the coagulation bath at the desired height and provision is also made to introduce and remove gaseous medium from the shaft area between the spinneret and the top of the coagulation bath if desired.